Recently, municipal and factory wastes have been steadily increasing and their disposal has become a social problem. Power generating units have been developed for recovery of combustion heat through incineration of such wastes or through use of refuse derived fuel.
One type of such power generating units is a circulating fluidized bed combustor which comprises, as shown in FIG. 1, a combustion furnace 1 for burning wastes such as municipal waste or fuel such as refused derived fuel while fluidizing them together with bed material 3 such as sand or limestone through primary air A blown through an air dispersion nozzle 2, a hot cyclone 4 as material separator connected to a top of the furnace 1 for capturing bed material such as ash or sand entrained in exhaust gas generated in combustion in the furnace 1, an external heat exchanger 7 as external recirculation unit into which introduced through a downcomer 5 is the bed material captured in the cyclone 4, said bed material being heated and returned through a bed material return pipe 6 into a bottom of the furnace 1or circulation, and a heat recovery part 10 into which introduced is the exhaust gas separated from the bed material in the cyclone 4 and which is internally provided with a superheater 8 and a fuel economizer 9.
Arranged in the heat recovery part 10 and downstream of the economizer 9 is a gas air heater 12 which heats air conveyed from a forced draft fan 11 through heat of the exhaust gas. The air heated by the heater 12 is fed as primary air A via a primary air line 13 to the bottom of the furnace 1, and is fed as secondary air B via a secondary air line 14 branched from the primary air line 13 sideways of the furnace 1. Air conveyed from a fluidizing air blower 15 is fed as fluidizing air C via a fluidizing air line 18 into a bottom of the heat exchanger 7. Incorporated in the primary air line 13 and downstream of the branch into the secondary air line 14 is a damper 16 for control in flow rate of the primary air A; incorporated in the secondary air line 14 is a damper 17 for control in flow rate of the secondary air B.
The external heat exchanger 7 is formed with a wind box 21 at a bottom of a seal box 19 into which the downcomer 5 is connected, so as to blow the fluidizing air C upwardly through an air dispersion nozzle 20. Arranged in the seal box 19 and above the nozzle 20 is a final superheater 22 which heat-exchanges with the bed material to generate and introduce superheated steam into a steam turbine. In view of the fact that generally the external heat exchanger 7 has higher pressure than a lower part of the combustion furnace 1 because of material-sealing below the downcomer 5 by the bed material, the external heat exchanger 7 is made in the form of so-called siphon for prevention of the exhaust gas in the furnace 1 from flowing into the lower part of the heat exchanger 7 or downcomer 5 and for reliable flow and return of the bed material separated in the cyclone 4 into the furnace 1.
In the above-mentioned circulating fluidized bed combustor as power generating unit, the air conveyed from the fan 11 and heated by the heater 12 is fed as primary air A via the line 13 into the bottom of the furnace 1 and fed as secondary air B via the line 14 branched from the line 13 sideways of the furnace 1, and the air conveyed from the blower 15 is fed as fluidizing air C via the line 18 into the bottom of the heat exchanger 7. In this state, waste such as municipal waste or refuse derived fuel is charged over the air dispersion nozzle 2 in the furnace 1 and is burned while being fluidized together with the bed material 3 through the primary air A blasted via the nozzle 2.
The exhaust gas generated by combustion of the waste in the furnace 1 is blown up together with the bed material such as ash or sand into the cyclone 4 where the bed material is captured. The bed material captured in the cyclone 4 is introduced through the downcomer 5 connected to the bottom of the cyclone 4 into the external heat exchanger 7 as external recirculation unit where the bed material is robbed of heat and returned through the return pipe 6 into the bottom of the furnace 1 for circulation.
The exhaust gas separated from the bed material in the cyclone 4 is guided to the heat recovery part 10 and is heat-recovered by the superheater 8 and economizer 9 in the heat recovery part 10 and further by the gas air heater 12, and then is passed through a dust collector or the like (not shown) and discharged through a flue to atmosphere.
Boiler feedwater is heated in the economizer 9 by the exhaust gas, caused to flow via a steam drum (not shown) into a furnace wall 1a of the furnace 1, returned again to the steam drum where it is turned out into saturated steam and guided to the superheater 8 The steam superheated by the exhaust gas and further superheated in the superheater 8 is guided to the final superheater 22 where the superheated steam is still further superheated by the bed material. The steam superheated in the superheater 22 is guided to the steam turbine where power generation is conducted.
In the above-mentioned circulating fluidized bed combustor, what amount a circulation quantity of the bed material is to be controlled to for uniformization and stabilization in temperature of the combustion furnace may be theoretically determined through systematic calculation or the like. In order to stabilize the operation in this manner, the circulation quantity of the bed material must be accurately grasped; conventionally, it has been conducted to qualitatively evaluate circulation quantity of bed material from pressure difference between combustion furnace 1 and hot cyclone 4.
Means for estimating circulation quantity of bed material has been disclosed, for example, in Reference 1.
[Reference 1] JP 2001-289406A